1. Field of the Invention
The present invention relates to a differential circuit, an output buffer and a semiconductor integrated circuit, and more particularly to a differential circuit, an output buffer and a semiconductor integrated circuit that can be used in a multi-power system.
2. Description of the Related Art
A power supply voltage used in a complementary metal-oxide semiconductor (CMOS) circuit has been decreasing according to the development of CMOS technology. Accordingly, it is becoming more difficult to provide a high output voltage at an output buffer using a conventional CMOS circuit.
FIG. 1 is a circuit diagram illustrating a conventional output buffer employing a transistor having a low-voltage gate oxide.
Referring to FIG. 1, the conventional output buffer circuit includes loads R11 and R12 coupled to a low power supply voltage VDDL, for example, about 1.2 volts, NMOS transistors NT11 and NT12 that function as a differential switching circuit, and an NMOS transistor NT13 that functions as a constant current source using a biasing control voltage Vc.
The output buffer circuit of FIG. 1 provides a low-voltage output signal using the low power supply voltage VDDL.
Particularly, the output buffer circuit of FIG. 1 receives two differential input voltages VI+ and VI− that swing between a first voltage level and a second voltage level, and provides differential output voltages VO+ and VO− that swing between a third voltage level and a fourth voltage level using the low power supply voltage VDDL.
The transistors NT11 and NT12 respectively are implemented with a low-voltage gate oxide transistor. The low-voltage gate oxide transistor includes a gate dielectric layer (for example, a gate oxide) having a thickness that can endure a voltage level of the low power supply voltage VDDL, for example, about 1.2 volts. That is, operation of the transistors NT11 and NT12 above a gate-body voltage VgbMAX significantly degrades the gate dielectric layer. A high-voltage gate oxide transistor includes a gate oxide having a thickness that can endure a voltage level of a high power supply voltage, for example, about 2.5 volts. The low-voltage gate oxide transistor may have a gate oxide that is relatively thin thickness compared with that of the high-voltage gate oxide transistor.
A body, i.e., a p-substrate, of the NMOS transistors NT11 and NT12 are coupled to a bias voltage of a ground level. Thus, a maximum voltage difference between a gate and a body of each of the transistors NT11 and NT12 is the low power voltage VDDL.
In the conventional output buffer circuit of FIG. 1, the NMOS transistors NT11 and NT12 are implemented with a low-voltage gate oxide NMOS transistor, of which a maximum allowable voltage is about 1.2 volts, and a high power supply voltage VDDH is coupled to the loads R11 and R12 so as to output a high-voltage output signal. Thus, a voltage difference Vgb between the gate and the body of each of the transistors NT11 and NT12 may be larger than the maximum allowable voltage of 1.2 volts of the low-voltage gate oxide NMOS transistor, and reliability of the thin gate oxide correspondingly is deteriorated.
Accordingly, since the reliability of the output buffer of FIG. 1 is deteriorated, the output buffer has to be implemented with a thick gate oxide transistor, i.e., a high-voltage transistor, as the differential switching transistor in order to operate at a high speed using a low-voltage NMOS transistor of a thin gate oxide, and simultaneously in order to obtain a high-voltage output signal by increasing a voltage level of the power supply voltage.
FIG. 2 is a circuit diagram illustrating a conventional output buffer circuit employing a transistor having a high-voltage gate oxide.
Referring to FIG. 2, the output buffer circuit includes loads R21 and R22 coupled to a high power supply voltage VDDH, for example, about 2.5 volts, NMOS transistors NT21 and NT22 that function as a differential switching circuit, and an NMOS transistor NT23 that functions as a constant current source.
The output buffer circuit of FIG. 2 provides a high-voltage output signal using the high power supply voltage VDDH.
Particularly, the output buffer circuit of FIG. 2 receives two differential input voltages VI+ and VI−, and provides differential output voltages VO+ and VO−, of which a maximum voltage level is substantially the same as a level of the high power supply voltage, using the high power supply voltage VDDH.
The transistors NT21 and NT22 respectively include a high-voltage gate oxide transistor that includes a gate oxide having a thickness that can endure a voltage level of the high power supply voltage VDDH, for example, about 2.5 volts. Bodies of the NMOS transistors NT21 and NT22 are coupled to a bias voltage of a ground level. Thus, a maximum voltage difference between a gate and a body of each of the transistors NT11 and NT12 is the high power supply voltage VDDH.
The thick gate oxide transistor cannot provide high operational speed due to relatively low driving capacity, compared with the thin gate oxide transistor.
When the NMOS transistors NT21 and NT22 employ a low-voltage gate oxide NMOS transistor in an output buffer circuit that operates at the high power supply voltage VDDH, the maximum voltage difference between the gate and the body of each of the transistors NT11 and NT12 may be the high power supply voltage VDDH.
However, the reliability of the low-voltage gate oxide transistor may be deteriorated due to a bias voltage higher than the maximum allowable voltage of the low-voltage gate oxide transistor. Thus, it is difficult to employ the low-voltage gate oxide transistor in an output buffer circuit that operates at a high power supply voltage.
That is, it is difficult to obtain high operational speed when the high-voltage gate oxide transistor is employed in the output buffer circuit that operates at the high power supply voltage, and operational reliability may be deteriorated when the low-voltage gate oxide transistor is employed in the output buffer circuit that operates at the high power supply voltage so as to obtain the high operational speed.
Therefore, the conventional output buffer circuit that operates at the high power supply voltage so as to obtain the high-voltage output signal cannot provide the high operational reliability and the high operational speed at the same time. That is, the conventional output buffer circuit that operates at the high power supply voltage cannot provide simultaneously both of the high operational speed and the high-voltage output signal.